1. Field
The present invention relates generally to CDMA communication systems, and more specifically to a linear MMSE space-time equalizer for MIMO multi-code CDMA systems.
2. Background
In wireless communication systems, several users share a channel within a common spectrum. To avoid conflicts arising from several users transmitting information over the communication channel at the same time, some regulation on allocating the available channel capacity to the users is required. Regulation of user access to the communication channel is achieved by various forms of multiple access protocols. One form of protocol is known as code division multiple access (CDMA). In addition to providing multiple access allocation to a channel of limited capacity, a protocol can serve other functions. For example, a protocol can provide isolation of users from each other, limit interference between users, and provide security by making interception and decoding difficult for a non-intended receiver, also referred to as low probability of intercept.
In CDMA systems, each signal is separated from those of other users by coding the signal. The information signal is uniquely encoded into a transmission signal. The intended receiver, knowing the code sequences of the user, can decode the transmission signal to receive the information. The information signal spectrum is spread by a code so that the bandwidth of the encoded transmission signal is much greater than the original bandwidth of the information signal. For this reason, CDMA is a form of “spread spectrum” coding. The energy of each user's signal is spread across the channel bandwidth so that each user's signal appears as noise to the other users. So long as the decoding process can achieve an adequate signal to noise ratio, the information in the signal can be recovered, (separation of the desired user's signal from the “noise” of the other users' signals). Other factors, which affect information recovery of the user's signal, are different conditions in the environment for each subscriber, such as fading, shadowing and multipath. Shadowing is interference caused by a physical object interrupting the signal transmission path between the transmitter and receiver, for example, a large building. Multipath is a signal distortion, which occurs as a result of the signal traversing multiple paths of different lengths and arriving at the receiver at different times. Multipath is also referred to as “time dispersion” of the communication channel. Signals that are received in phase reinforce each other and produce a stronger signal at the receiver, while those that are received out of phase produce a weak or fading signal. Multipath fading may also vary with time. For example, in a communication unit being carried in a moving car, the amount of multipath fading can vary rapidly.
To provide diversity against deleterious path effects and improve performance, multiple transmit and receive antennas may be used. If the transmission paths between the transmit and receive antennas are linearly independent (i.e., a transmission on one path is not formed as a linear combination of the transmissions on other paths, which is generally true to some extent, then the likelihood of correctly receiving a transmitted signal increases as the number of antennas increases. Generally, diversity increases and performance improves as the number of transmit and receive antennas increases. The use of multiple antennas at the transmitter and the receiver is used in multiple input multiple output (MIMO) systems.
If multiple antennas are available at the transmitter or the receiver, the peak throughput can be increased using techniques, such as spatial multiplexing and code-reuse. With code re-use, each channel allocated for transmission can modulate up to M distinct data streams, where M is the number of transmit antennas. Data streams, which share the same code, are distinguished based on their spatial characteristics, requiring a receiver with at least M antennas. In principle, the peak throughput with code re-use is M times the rate achievable with a single antenna.
In MIMO multi-code CDMA systems, reuse of the same spreading codes in different transmit antennas degrades the equalization performance if the space-time equalizer uses a minimum mean squared error (MMSE) weighting vector that minimizes the mean squared error of the equalizer output chip sequence. The CDMA despreader distorts inter-stream interference components differently from multipath interference and background noise components. This degrades performance of prior art MIMO systems.
There is therefore a need in the art for an enhanced chip-level linear space-time equalizer for multiple-input-multiple-output (MIMO) multi-code CDMA systems that can reuse spreading codes in different transmit antennas.